Small engine Direct Drive Prop question

[NocEm]

I am confused on prop selection.

Which one of these props used direct drive would push more air. The motor is a Briggs Vangaurd 35 HP, like the mini airboats run.


1. large diameter prop pitched flat to acheive 2800 RPM

2. Smaller diameter prop pitched more aggresive to achive 2800 RPM.

I hope all this makes sense.

I imagine the theory is the same on these smaller engines compared to say a bigger car motor running direct drive.

 

[Skeeter]

I have always heard bigger is better.But I'm not an expert.But if you have no pitch with the larger prop, I would say go to a smaller prop to acheive pitch.I could be wrong.

 

[nebraskaairboater]

I was of the understanding that you always want to swing the larges diameter prop you can fit in your cage. The reasoning is that you get more surface area for push. The longer the prop the bigger the "wing".
The longer prop will give more push and the shorter prop with more pitch might give more top end.

This is how it was explained to me but I'm sure someone else on here is more qualified to give an answer.

Also you might not need 2800 rpm depending on your prop. I run mine at 2600 wide open and it pushes great. Cruise at 2000 or 2100 direct drive

 

[NocEm]

I need to be more specific I guess,

I am not concerned about speed the only thing that I care about is low end push/torque. I should have said that earlier. I hope this helps. Thanks for the help all.

 

[Daddy Dave]

First, you have to find out what B&S says the safe max RPM is for TORQUE. TORQUE = PUSH. Then load your boat as you intend to use it and then set the prop to attain max. torque at the prescribed load and suggested RPM.

 

[NocEm]

I got that,

Max torque is 2400 RPM, max HP is 3600 RPM. I ran my last rig with a 3 blade 50 inch adjustable prop at all ranges in between and it seemed to run the best at 2800 rpm coincidently right where both the torque curve and the HP curve intersected each other. I am in the process of building a new rig and am trying to figure out, with y'alls help what combination I need to attain optimum performamance.

 

[Daddy Dave]

Most props stopp adding push at about 2,900. It would seem that you could use a compact reduction unit to maximize your torque - (Hear that Waterthunder?

 

[NocEm]

There are a few small shops that make a belt drive for the Briggs, but the dependabilty of the belts concerns me. I might contemplate a gear box if someone ever made of one, but until that day I will just have to make due direct.

 

[jim]

I believe Paul Dixon can answer your questions:

http://airboatsbydixon.com/

jim

 

[Whitebear]

There are a few small shops that make a belt drive for the Briggs, but the dependabilty of the belts concerns me.

Theres a LOT of those belt drives flying in both experimental and registered aircraft and gyroplanes. Reliability seems to not be an issue at all when properly designed. Check out some of the rotorcraft forums or home built planes sites.

Scotty

 

[leo_081103]

Hello Guys,

Maybe I can help here. First of all, you don't need to have a reduction drive for that B&S 35hp engine IF you dont intend to change your prop. A 50 inch prop will make a tip speed of 785.4 fps, a good tip speed to consider. Secondly, on prop design, the best is to install the greatest diameter possible PROVIDED that you dont compromise tip speed, pitch.

Please note that torque is always accompanied with power. The key is power and not torque! Any prop designed for highest torque will perform less to a prop desiged from power...parameters being the same.

At one extreme end, IF you plan to use your rig as a tug, say at 1 mph, probably, use a 92 in x 12 in @ 120 rpm or so. On the other extreme end, you may think of increasing rpm say 2:1 or twice the 3600 rpm rating resulting to a prop of 24in dia with a pitch so high you end up slapping the air rather than cutting through it ( technically called "stalled blades" ) & you also go nowhwere at all.

I hope you are now aware of the two ends of prop sizing.

If you want, please send me or post the largest prop that will fit your cage . Please clear your tips of the structure by 2" i.e. max. diameter of cage less 4 inches and I will advise you on the set up and some options.

Happy Airboating Everyone!

 

[leo_081103]

By the way... it's not that difficult to design low powered belted reduction drives. So, going for a larger prop through a reduction drive is also a good option.